Total reflection spectroscopic measurement for measuring an object to be measured arranged on a total reflection surface of a prism using a terahertz wave is conventionally known. For example, as disclosed in Non-Patent Literature 1 (H. Hirori, K. Yamashita, M. Nagai, and K. Tanaka, “Attenuated total reflection spectroscopy in time domain using terahertz coherent pulses,” Jpn. J. Appl. Phys. 43, L1287, (2004)), a signal measured in a state in which the object to be measured is not arranged on the total reflection surface of the prism is used as a reference in conventional total reflection spectroscopic measurement. Then, a signal obtained by measuring the object to be measured is compared with the reference and an optical constant of the object to be measured is derived from a degree of change in the two signals.